1. Field of the Invention
The present invention relates to a drive circuit for a display apparatus which operates in response to digital video signals, and more particularly to a drive circuit for a display apparatus whose pixels need to be driven by an a. c. voltage and are liable to deterioration or breakage if they are driven by a d.c. voltage, such as a liquid crystal display (LCD) apparatus.
2. Description of the Prior Art
Hereinafter, a TFT (thin film transistor) liquid crystal display (LCD) apparatus will be described as a typical example of a type of a display apparatus in which the driving circuit of the present invention can be used.
FIG. 6 shows a source driver which is a part of a drive circuit for a TFT liquid crystal display apparatus. Digital video signal data are input to the source driver. In this example, it is assumed that the input digital video signal data consist of two bits (D1, D0) having four distinct values "0" to "3". The source driver selects one of the gradation voltages V.sub.0 to V.sub.3 which are supplied by voltage supply section 1 according to each value of the input digital video signal data, and outputs the selected voltage to source lines O.sub.n.
FIG. 7 shows a circuit corresponding to nth output portion of the source driver shown in FIG. 6. The circuit includes a D-type flip-flop (sampling flip-flop) M.sub.SMP at a first stage and a flip-flop (holding flip-flop) M.sub.H at a second stage which are provided for receiving each bit of the digital video signal data, a decoder DEC, and analog switches ASW.sub.0 to ASW.sub.3 for electrically connecting lines from the voltage supply section 1 with source lines O.sub.n. For the sampling of the digital video signal data, various circuits are available as alternatives to the D-type flip-flop.
The digital source driver shown in FIG. 7 operates as follows:
The sampling flip-flop M.sub.SMP latches the digital video signal data (D1, D0) at the rising edge of a sample pulse T.sub.SMPn corresponding to the nth pixel. When the sampling for one horizontal period is completed, an output pulse OE is fed to the holding flip-flop M.sub.H. Then, the data (D1, D0) held in the sampling flip-flop are moved to the holding flip-flop M.sub.H and are simultaneously output to the decoder DEC. The decoder DEC decodes the 2-bit data (D1, D0) and recognizes a value of the data (D1, D0). In accordance with the value of the data (D1, D0), the decoder DEC makes one of the analog switches ASW.sub.0 to ASW.sub.3 conductive. As a result, the corresponding one of four gradation voltages V.sub.0 to V.sub.3 is output to the source line O.sub.n.
FIG. 8 shows waveforms of the gradation voltages V.sub.0 to V.sub.3, and a common electrode voltage V.sub.COM applied to a common electrode for a LCD panel. The gradation voltages V.sub.0 to V.sub.3 are higher in this order, and are applied to the pixels. This relationship is expressed as follows: EQU .vertline.V.sub.0 -V.sub.COM .vertline..ltoreq..vertline.V.sub.1 -V.sub.COM .vertline..ltoreq..vertline.V.sub.2 -V.sub.COM .vertline..ltoreq..vertline.V.sub.3 -V.sub.COM .vertline., where the reverse relationship can also be used.
As shown in FIG. 8, the gradation voltages V.sub.0 to V.sub.3 and the common electrode voltage V.sub.COM alternately change between two voltage levels synchronously with a signal POL which is reversed for each output period. Alternatively, the common electrode voltage V.sub.COM may be d.c. voltage. Each level of the gradation voltages is determined to be symmetrical with respect to a given voltage (a central voltage) VM.
FIG. 9 shows each level of the gradation voltages V.sub.0 to V.sub.3 seen from a common electrode to which the common electrode voltage V.sub.COM is applied.
A particular pixel is charged with one of the gradation voltages shown in FIG. 9 when the pixel is selected by a gate driver (a scanning driver). If the pixel is selected at a beginning of a horizontal period when the gradation voltage is positive (i.e. the gradation voltage is higher than the common electrode voltage V.sub.COM) and the positive gradation voltage is applied to the selected pixel during the horizontal period, then it is controlled so that a negative gradation voltage (i.e. the gradation voltage is lower than the common electrode voltage V.sub.COM) corresponding to the positive voltage is applied to the selected pixel during the next horizontal period.
Thus, each pixel is changed with the gradation voltage which alternately changes between a positive voltage level and a negative voltage level, that is, an a.c. voltage, resulting in preventing a d.c. voltage from being applied to the pixel as an average value.
Under an ideal condition, the known drive circuit mentioned above protects the pixels against breakage or deterioration due to the application of d.c. voltage. However, an actual liquid crystal display apparatus can not be completely protected in such a manner because of the fact that the voltages applied to the pixels in a liquid crystal display panel are not the same as the gradation voltages V.sub.0 to V.sub.3 and the common electrode voltage V.sub.COM. The cause of this voltage difference is explained as follows:
FIGS. 12, 13 and 14 show equivalent circuits of a pixel portion including a pixel capacitance C.sub.LC and an additional capacitance C.sub.S which are connected in parallel to a common electrode COM. In FIG. 12, C.sub.gd represents a parasitic capacitance present between the gate and drain of a thin film transistor (TFT) 10.
When the voltage of the gate line G.sub.n is high (V.sub.GH), the TFT 10 is turned on. As a result, a voltage V.sub.S of the source line O.sub.n is applied to the pixel. Then, the following equations are established among charges q.sub.1, q.sub.2 and q.sub.3 in the capacitances shown in FIG. 13: EQU q.sub.1 +q.sub.2 +q.sub.3 =constant EQU q.sub.1 /C.sub.LC =q.sub.2 /C.sub.S =V.sub.S EQU q.sub.3 =C.sub.gd .multidot.(V.sub.S -V.sub.GH)
Accordingly, the following equation (1) is obtained: EQU (C.sub.LC +C.sub.S +C.sub.gd).multidot.V.sub.S =constant+C.sub.gd .multidot.V.sub.GH ( 1)
On the other hand, when the voltage of the gate line G.sub.n is low (V.sub.GL), the TFT 10 is turned off. As a result, the following equations are established among charges q.sub.1 ', q.sub.2 ' and q.sub.3 ' in the capacitances shown in FIG. 14: EQU q.sub.1 '+q.sub.2 '+q.sub.3 '=constant EQU q.sub.1 '/C.sub.LC =q.sub.2 '/C.sub.S =V.sub.S ' EQU q.sub.3 '=C.sub.gd .multidot.(V.sub.S '-V.sub.GL)
Accordingly, the following equation (2) is obtained: EQU (C.sub.LC +C.sub.S +C.sub.gd).multidot.V.sub.S '=constant+C.sub.gd .multidot.V.sub.GL ( 2)
From the equations (1) and (2), the following equation (3) is derived. EQU V.sub.S -V.sub.S '=C.sub.gd .multidot.(V.sub.GH -V.sub.GL)/(C.sub.LC +C.sub.S +C.sub.gd) (3)
As is evident from the foregoing equations, while the voltage of the gate line G.sub.n is high (V.sub.GH), the pixel is charged with the voltage V.sub.S, and after the TFT 10 turns off, the voltage V.sub.S is varied into a voltage V.sub.S '. The difference between V.sub.S and V.sub.S ' is represented by equation (3).
The difference V.sub.S -V.sub.S ' is observed as a variation of the pixel characteristics caused by the application of a positive and a negative gradation voltages to the pixels, when the positive and negative gradation voltages are actually applied to the driving terminal of a LCD panel. As a result, some of d.c. voltage components are applied to the pixels in a LCD panel according to the driving voltages shown in FIG. 8, which is described below in more detail.
FIG. 10 shows an exemplary relationship between a gradation voltage (an absolute value) input to a LCD panel and transmissivity characteristics of the pixels in a LCD panel. The gradation voltage is applied to the driving terminal of a LCD panel. In FIG. 10, a scale of the horizontal axis is determined so that a relationship between an absolute value of a negative gradation voltage and the transmissivity characteristics is represented as a straight line.
The V.sub.N.sup.+ and V.sub.N.sup.- (where N=0, 1, 2, 3) represent positive gradation voltages and negative gradation voltages which are required to achieve the same transmissivity characteristics of pixels in a LCD panel respectively. For example, V.sub.3.sup.+ and V.sub.3.sup.-, as well as V.sub.0.sup.+ and V.sub.0.sup.-, which are required to achieve the same transmissivity characteristics of the pixel in a LCD panel are shown in FIG. 10. The positive and negative voltages are defined by the differences between the gradation voltages V.sub.N and the common electrode voltage V.sub.COM, as mentioned above. .DELTA.V.sub.N represents a difference between the voltages V.sub.N.sup.+ and V.sub.N.sup.-.
FIG. 11 shows a relationship between a negative gradation voltage (an absolute value) applied to the driving terminal of a LCD panel and the difference .DELTA.V.sub.N. FIG. 11 teaches that a positive gradation voltage V.sub.N.sup.+ should be substantially equal to a voltage obtained by adding the difference .DELTA.V.sub.N to an absolute value of a negative gradation voltage V.sub.N.sup.- in order to achieve the same transmissivity characteristics of the pixel.
For example, a case where the gradation voltage is V.sub.3 (N=3) will be described below. Assuming that a LCD panel is driven with the gradation voltage which has a waveform shown in FIG. 9, and that an absolute value of the difference between the gradation voltage V.sub.3 and the common electrode voltage V.sub.COM in FIG. 9 is substantially equal to a voltage represented by V.sub.3 in FIG. 10. In such a case, a difference .DELTA.t.sub.3 in the transmissivity characteristics occurs depending on which voltage is applied to a pixel, a positive gradation voltage or a negative gradation voltage, as shown in FIG. 10.
If the absolute value of the positive and negative gradation voltage levels actually applied to the pixels are different from each other, the pixel characteristics (for example, the transmissivity of liquid crystal in a case where a LCD panel is used) are varied, even though the display apparatus is not broken or deteriorated. The variation of the pixel characteristics causes a flicker effect of the image.
Further, the application of a d.c. voltage to the pixels causes not only a deterioration of the display apparatus but also a so-called "after static image" problem where a static image remains visible in the display even after the static image is deleted. In the display apparatus in which a static image is the primary mode of display, such as a terminal display device for a computer, the problem is serious.